Synthetic aperture radar imaging apparatus and methods

ABSTRACT

A synthetic aperture radar (SAR) is operable in an interrogation mode and in an imaging mode, the imaging mode entered in response to determining a response to interrogation pulses have been received from a ground terminal and position information specifying a ground location has been received from the ground terminal. A ground terminal is operable to receive interrogation pulses transmitted by a SAR, transmit responses, and transmit position information to cause the SAR to enter a imaging mode. The ground terminal receives first and subsequent pulses from the SAR where subsequent pulses include backscatter and are encoded. The ground terminal generates a range line by range compression. If the SAR is a multi-band SAR the transmitted pulses can be in two or more frequency bands, and subsequent pulses in one frequency band can include encoded returns from pulses transmitted in a different frequency band.

TECHNICAL FIELD

The present application relates generally to synthetic aperture radar (SAR) and, more particularly, to operating modes suitable for situational awareness.

BACKGROUND Description of the Related Art

A synthetic aperture radar (SAR) is an imaging radar. The SAR exploits the relative motion of the radar and a target of interest to obtain high azimuthal resolution. High range resolution can be achieved using pulse compression techniques. The SAR is typically flown on an aircraft, a spacecraft, unmanned aerial vehicle (UAV) such as a drone, or another suitable platform. The target of interest is typically on the ground, and can be a point target or a distributed target. The SAR can be a component of a SAR imaging system, the system also including at least one of data processing and data distribution components.

In conventional operation of the SAR imaging system, the system is tasked to obtain images of a target or a swath. Data is collected on-board the platform. In the case of a spaceborne SAR, the data is collected on-board the spacecraft, and either processed on-board the spacecraft and downlinked to the ground, or downlinked and processed on the ground to generate the images. The images are distributed to the user, typically via a network.

BRIEF SUMMARY

A method of operation of a multi-band synthetic aperture radar (SAR) comprising a transceiver and a hardware data processor may be summarized as including causing by the data processor the SAR to enter an interrogation mode, and while in the interrogation mode: transmitting by the transceiver one or more interrogation pulses to the ground at at least one of a first or a second frequency band, the second frequency band different from the first frequency band; determining by the data processor if a response to the one or more interrogation pulses has been received from a ground terminal; determining by the data processor if position information specifying a ground location has been received from the ground terminal; upon determining by the data processor a response to the one or more interrogation pulses has been received from the ground terminal, and position information specifying a ground location has been received from the ground terminal, causing by the data processor the SAR to enter an imaging mode, and while in the imaging mode: transmitting by the transceiver a first transmitted imaging pulse to the ground at the first frequency band; transmitting by the transceiver a second transmitted imaging pulse to the ground at the second frequency band; receiving by the transceiver a first received pulse that includes backscattered energy from the first transmitted imaging pulse; receiving by the transceiver a second received pulse that includes backscattered energy from the second transmitted imaging pulse; encoding by the data processor data from the first and the second received pulses to generate a first subsequent transmitted imaging pulse; and transmitting by the transceiver the first subsequent transmitted imaging pulse to the ground at the first frequency band. Encoding by the data processor data from the first and the second received pulses to generate a subsequent transmitted pulse may include modulating by the data processor a combination of data from the first and the second received pulses by at least one of phase-shift keying (PSK) or amplitude and phase-shift keying (APSK) to generate a modulated pulse; convolutionally encoding by the data processor the modulated pulse to generate an encoded pulse.

Encoding by the data processor data from the first and the second received pulses to generate a subsequent transmitted pulse may further include encrypting by the data processor the encoded pulse to generate an encrypted subsequent transmitted pulse. Transmitting by the transceiver one or more interrogation pulses to the ground at at least one of a first or a second frequency band may include transmitting by the transceiver one or more pulses in a broad-beam mode of the multi-band SAR. Transmitting by the transceiver one or more interrogation pulses in a broad-beam mode of the multi-band SAR may include transmitting by the transceiver one or more pulses at a pulse repetition frequency in a range of 0.5 Hz to 50 Hz.

The method may further include transmitting by the transceiver a second subsequent transmitted pulse to the ground at the second frequency band; receiving by the transceiver a first subsequent received pulse at the first frequency band that includes backscattered energy from the first subsequent transmitted pulse; receiving by the transceiver a second subsequent received pulse at the second frequency band that includes backscattered energy from the second subsequent transmitted pulse; encoding by the data processor data from the first and the second subsequent received pulses to generate a further subsequent transmitted pulse; and transmitting by the transceiver the further subsequent transmitted pulse to the ground at the first frequency band.

The method may further include transmitting by the transceiver a third transmitted pulse to the ground at the first frequency band before transmitting by the transceiver the first subsequent transmitted pulse to the ground at the first frequency band, wherein the transmitting of the first subsequent transmitted pulse to the ground at the first frequency band does not consecutively follow the transmitting of the first transmitted pulse to the ground at the first frequency band. The multi-band SAR may be a dual-band SAR, and the first frequency band may be L-band and the second frequency band may be X-band. The transmitting of pulses at the first frequency band may be synchronized with the transmitting of pulses at the second frequency band. The transmitting of pulses at the first frequency band may be at a pulse repetition frequency that is substantially the same as for the transmitting of pulses at the second frequency band. The transmitting of pulses at the first frequency band may be interleaved with the transmitting of pulses at the second frequency band.

A method of generating synthetic aperture radar (SAR) images by a ground terminal, the ground terminal comprising a transceiver, a data store, and a hardware data processor may be summarized as including receiving by the transceiver an interrogation pulse transmitted by a multi-band SAR at at least one of a first or a second frequency band, the second frequency band different from the first frequency band; transmitting by the transceiver a response to the interrogation pulse; transmitting by the transceiver position information specifying a ground location; receiving by the transceiver an n^(th) transmitted first-frequency pulse transmitted by the multi-band SAR at the first frequency band; storing the n^(th) transmitted first-frequency pulse in the data store; receiving by the transceiver an m^(th) transmitted pulse transmitted by the multi-band SAR at the first frequency band, wherein m is greater than n, and wherein a radar return received by the multi-band SAR includes backscattered energy from the n^(th) transmitted first-frequency pulse and backscattered energy from an n^(th) transmitted second-frequency pulse transmitted by the multi-band SAR at the second frequency band, and the m^(th) transmitted pulse transmitted by the multi-band SAR at the first frequency band includes an encoding of the radar return received by the multi-band SAR; storing the m^(th) transmitted pulse in the data store; decoding by the data processor the m^(th) transmitted pulse to regenerate the radar return received by the multi-band SAR; and generating a range line by the data processor for a SAR image by range compression of at least a portion of the radar return received by the SAR using a reference function. The generating a range line by the data processor for a SAR image may include using the n^(th) transmitted first-frequency pulse as a reference function. The generating a range line by the data processor for a SAR image may include using a previously transmitted second-frequency pulse, transmitted by the multi-band SAR at the second frequency band, as a reference function.

The method may further include receiving the previously transmitted second-frequency pulse by the ground terminal. Decoding by the data processor the m^(th) transmitted pulse to regenerate the radar return received by the multi-band SAR may include convolutionally decoding by the data processor the m^(th) transmitted pulse to generate a modulated radar return; and demodulating by the data processor the modulated radar return to regenerate the radar return received by the multi-band SAR. The encoding of the m^(th) transmitted pulse may include an encryption of the radar return received by the multi-band SAR, and w decoding the m^(th) transmitted pulse by the data processor to regenerate the radar return received by the multi-band SAR may include decrypting by the data processor the m^(th) transmitted pulse to generate an encoded radar return; convolutionally decoding by the data processor the encoded radar return to generate a modulated radar return; and demodulating by the data processor the modulated radar return to regenerate the radar return received by the SAR. Transmitting by the transceiver position information specifying a ground location may include transmitting by the transceiver position information that includes a location of the ground terminal. Transmitting by the transceiver position information specifying a location to be imaged may include transmitting by the transceiver position information that includes the center of a desired image ground area. The multi-band SAR may be a dual-band SAR, and the first frequency band may be L-band and the second frequency band may be X-band.

A synthetic aperture radar (SAR) imaging system may be summarized as including a multi-band SAR mounted on a SAR platform, the multi-band SAR comprising a SAR antenna that transmits a first and a subsequent transmitted pulse to the ground at a first frequency band, and receives a first received pulse that includes backscattered energy from the first transmitted pulse, and transmits a second transmitted pulse to the ground at a second frequency band, and receives a second received pulse that includes backscattered energy from the second transmitted pulse, the second frequency band different from the first frequency band; a SAR transceiver communicatively coupled to the SAR antenna, the SAR transceiver communicatively coupled to at least one of a SAR data store and a SAR data processor, the SAR data store communicatively coupled to the SAR data processor, the SAR data store which stores the first and the second received pulses, and the SAR data processor which encodes data from the second received pulse to generate the subsequent transmitted pulse for transmission by the SAR antenna to the ground at the first frequency band. The SAR data processor may encode data from the first and the second received pulse to generate the subsequent transmitted pulse for transmission by the SAR antenna to the ground at the first frequency band.

The multi-band SAR selectively operable in an interrogation mode to transmit a plurality of interrogation pulses to a ground at at least one of a first or a second frequency band, may further include a ground terminal, the ground terminal comprising a ground terminal antenna communicatively coupled to a transceiver, the transceiver communicatively coupled to at least one of a ground terminal data store and a ground terminal data processor, wherein the ground terminal is selectively operable to receive an interrogation pulse and respond by transmitting position information specifying a ground location to the SAR, and wherein the SAR imaging system, in response to receiving position information from the ground terminal, generates one or more SAR images of the ground in response to the interrogation pulse at at least one of a first or a second frequency band. The SAR antenna may be a planar phased array antenna.

A method of operation of a radar comprising a transmitter, a receiver, and a set of processor circuitry may be summarized as including transmitting by the transmitter a first pulse at a first frequency band; transmitting by the transmitter a second pulse at a second frequency band, the second frequency band different from the first frequency band; receiving by the receiver a first radar return that includes backscattered energy from the first pulse; receiving by the receiver a second radar return that includes backscattered energy from the second pulse; encoding by the set of processor circuitry data from the first and the second radar returns to generate a subsequent pulse; and transmitting by the transmitter the subsequent pulse at the first frequency band. Receiving by the receiver a radar return that includes backscattered energy from the first pulse may include receiving by the receiver a radar return that includes backscattered energy from the first pulse which is backscattered from a target on a surface of the Earth. The radar may be operable to image the target on the surface of the Earth from at least one of an airborne platform or a spaceborne platform. The radar may include a transceiver, the transceiver including the transmitter and the receiver.

Encoding by the set of processor circuitry data from the first and the second radar returns to generate a subsequent pulse may include sampling data from the first and the second radar returns to generate sampled radar return; modulating by the set of processor circuitry the sampled radar return by at least one of phase-shift keying (PSK) or amplitude and phase-shift keying (APSK) to generate modulated radar returns; and convolutionally encoding by the set of processor circuitry the modulated radar returns to generate an encoded radar return.

Encoding by the processor circuitry data from the first and the second radar returns to generate a subsequent pulse may further include encrypting by the set of processor circuitry the encoded radar return to generate an encrypted subsequent pulse. The radar may be a multi-band synthetic aperture radar.

A method of operation of a multi-band synthetic aperture radar (SAR) comprising a transceiver and a hardware data processor may be summarized as including causing by the data processor the SAR to enter an interrogation mode, and while in interrogation mode; transmitting by the transceiver one or more interrogation pulses to a ground at at least one of a first or a second frequency band, the second frequency band different from the first frequency band; determining by the data processor if a response to the one or more interrogation pulses has been received from a ground terminal; determining by the data processor if position information specifying a ground location has been received from the ground terminal; upon determining by the data processor a response to the one or more interrogation pulses has been received from the ground terminal, and position information specifying a ground location has been received from the ground terminal, causing by the data processor the SAR to enter an imaging mode, and while in imaging mode; transmitting by the transceiver a first transmitted imaging pulse to the ground at the second frequency band; receiving by the transceiver a first received pulse that includes backscattered energy from the first transmitted imaging pulse; encoding by the data processor data from the first received pulse to generate a first subsequent transmitted imaging pulse; and transmitting by the transceiver the first subsequent transmitted imaging pulse to the ground at the first frequency band.

The method may further include transmitting by the transceiver a second subsequent transmitted imaging pulse to the ground at the second frequency band; receiving by the transceiver a subsequent received pulse at the second frequency band that includes backscattered energy from the second subsequent transmitted imaging pulse; encoding by the data processor data from the subsequent received pulse to generate a further subsequent transmitted pulse; and transmitting by the transceiver the further subsequent transmitted pulse to the ground at the first frequency band.

A method of generating synthetic aperture radar (SAR) images of a ground by a ground terminal, the ground terminal comprising a transceiver, a data store, and a hardware data processor may be summarized as including receiving by the transceiver an interrogation pulse transmitted by a multi-band SAR at a first frequency band; transmitting by the transceiver a response to the interrogation pulse; transmitting by the transceiver position information specifying a ground location; receiving by the transceiver an n^(th) transmitted first-frequency pulse transmitted by the multi-band SAR at the first frequency band; storing the n^(th) transmitted first-frequency pulse in the data store; receiving by the transceiver an m^(th) transmitted pulse transmitted by the multi-band SAR at the first frequency band, wherein m is greater than n, and wherein a radar return received by the multi-band SAR includes backscattered energy from an n^(th) transmitted second-frequency pulse transmitted by the multi-band SAR at a second frequency band, the second frequency band different from the first frequency band, and the m^(th) transmitted pulse transmitted by the multi-band SAR at the first frequency band includes an encoding of data from the radar return received by the multi-band SAR; storing the m^(th) transmitted pulse in the data store; decoding by the data processor the m^(th) transmitted pulse to regenerate the radar return received by the multi-band SAR; and generating a range line by the data processor for a SAR image by range compression of at least a portion of the radar return received by the SAR using a reference function. Generating a range line by the data processor for a SAR image by range compression of at least a portion of the radar return received by the SAR may include generating a range line by the data processor for a SAR image by range compression of at least a portion of the radar return received by the SAR using a previously transmitted second-frequency pulse transmitted by the multi-band SAR at the second frequency band as the reference function.

The method may further include receiving the previously transmitted second-frequency pulse by the ground terminal. Receiving the previously transmitted second-frequency pulse by the ground terminal may include receiving a previously transmitted first-frequency pulse and decoding the previously transmitted first-frequency pulse to generate the reference function.

The multi-band SAR may be a dual-band SAR, and the first frequency band may be L-band and the second frequency band may be X-band.

A method of operation of a radar comprising a transmitter, a receiver, and a set of processor circuitry may be summarized as including transmitting by the transmitter a pulse at a second frequency band; receiving by the receiver a radar return that includes backscattered energy from the pulse at the second frequency band; encoding by the set of processor circuitry data from the radar return to generate a subsequent pulse at a first frequency band, the second frequency band different from the first frequency band; and transmitting by the transmitter the subsequent pulse at the first frequency band.

A multi-band synthetic aperture radar (SAR) system may be summarized as including a SAR antenna, SAR transceiver communicatively coupled to the SAR antenna, a SAR data processor and a SAR data store, the SAR transceiver communicatively coupled to at least one of a SAR data store and a SAR data processor; and a ground terminal comprising a ground terminal transceiver, a data store, and a hardware data processor communicatively coupled to the ground terminal transceiver and the data store; wherein the multi-band synthetic aperture radar SAR system is selectively operable to perform the method of any of claims 1 through 20 and 25 through 32.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not necessarily drawn to scale, and some of these elements may be arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not necessarily intended to convey any information regarding the actual shape of the particular elements, and may have been solely selected for ease of recognition in the drawings.

FIG. 1A is a schematic diagram illustrating the illumination geometry of an example embodiment of a SAR imaging system in accordance with the present systems, devices, methods, and articles.

FIG. 1B is a schematic diagram illustrating the illumination geometry of another example embodiment of a SAR imaging system in accordance with the present systems, devices, methods, and articles.

FIG. 2 is a block diagram illustrating an example embodiment of a SAR imaging system in accordance with the present systems, devices, methods, and articles.

FIG. 3 is a schematic diagram illustrating operation of an example embodiment of a SAR imaging system in accordance with the present systems, devices, methods, and articles.

FIG. 4 is a timing diagram illustrating an example timing sequence for operation of a SAR imaging system in accordance with the present systems, devices, methods, and articles.

FIG. 5 is a flow chart illustrating a method of operation of a SAR imaging system in accordance with the present systems, devices, methods, and articles.

FIG. 6 is a timing diagram illustrating an example timing sequence for transmitted and received pulses during operation of a SAR imaging system in accordance with the present systems, devices, methods, and articles.

FIG. 7 is a flow chart illustrating an example method of encoding a transmitted pulse of a SAR imaging system in accordance with the present systems, devices, methods, and articles.

FIG. 8 is a flow chart illustrating another example method of encoding a transmitted pulse of a SAR imaging system in accordance with the present systems, devices, methods, and articles.

FIG. 9 is a flow chart illustrating a method of range compressing received pulses by a ground terminal in accordance with the present systems, devices, methods, and articles.

FIG. 10 is a flow chart illustrating another method of range-compressing received pulses by a ground terminal in accordance with the present systems, devices, methods, and articles.

FIG. 11 is a flow chart illustrating an example method of forming a self-image by a ground terminal in accordance with the present systems, devices, methods, and articles.

FIG. 12 is a flow chart illustrating another example method of encoding a transmitted pulse of a radar in accordance with the present systems, devices, methods, and articles.

FIG. 13 is a flow chart illustrating another example method of range-compressing received pulses by a ground terminal of a radar in accordance with the present systems, devices, methods, and articles.

FIG. 14 is a schematic diagram illustrating the illumination geometry of an example embodiment of a dual-band SAR imaging system in accordance with the present systems, devices, methods, and articles.

FIG. 15 is a flow chart illustrating an example method of encoding a transmitted pulse of a multi-band radar in accordance with the present systems, devices, methods, and articles.

FIG. 16 is a flow chart illustrating another example method of encoding a transmitted pulse of a multi-band radar in accordance with the present systems, devices, methods, and articles.

DETAILED DESCRIPTION

The technology described in this application is a SAR mode for applications requiring real-time or near real-time situational awareness on demand, such as search and rescue. It is particularly suitable for circumstances where a person in the field needs to know what is in the immediate vicinity, say, in a local 5 km to 10 km square area. The technology is suitable for a SAR imaging system mounted on a spacecraft, an aircraft or a drone, for example. The person in the field requires a ground terminal for receiving signals from the SAR and for transmitting signals to the SAR.

Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.”

Reference throughout this specification to “one implementation” or “an implementation” or “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the implementation or embodiment is included in at least one implementation or at least one embodiment. Thus, the appearances of the phrases “one implementation” or “an implementation” or “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same implementation or the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more implementations or one or more embodiments.

As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its broadest sense, that is as meaning “and/or” unless the content clearly dictates otherwise.

The Abstract of the Disclosure and headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

FIG. 1A is a schematic diagram illustrating the illumination geometry of an example embodiment of a SAR imaging system in accordance with the present systems, devices, methods, and articles. The SAR imaging system of FIG. 1A comprises a spaceborne platform 110 a and a ground terminal 120. Spaceborne platform 110 a comprises a synthetic aperture radar (not shown in FIG. 1A). The elements of the synthetic aperture radar (SAR) are described in reference to FIG. 2 (below).

Spaceborne platform 110 a can be, for example, a satellite, a spacecraft, or a space station. In some implementations, spaceborne platform 110 a can be replaced by an aircraft or an unmanned aircraft such as a drone, for example (see FIG. 1B). In some implementations, ground terminal 120 is mobile. In other implementations, ground terminal 120 is fixed. In some implementations, ground terminal 120 is man-portable.

Spaceborne platform 110 a flies along trajectory 130. Dashed line 140 indicates a ground track of spaceborne platform 110 a. Line 150 and line 155 indicate a near-side and a far-side of a swath, respectively. Shaded region 160 represents a main lobe of a SAR antenna beam pattern on the ground. As spaceborne platform 110 a flies along trajectory 130, ground terminal 120 will first enter and then leave the SAR antenna beam, as represented by shaded region 160.

FIG. 1B is a schematic diagram illustrating the illumination geometry of another example embodiment of a SAR imaging system in accordance with the present systems, devices, methods, and articles using an airborne platform 110 b instead of spaceborne platform 110 a. Airborne platform 110 b can be, for example, an aircraft, an unmanned aircraft, or a drone.

The SAR imaging system of FIGS. 1A and 1B can be a side-looking radar, and can be right-looking (as in FIG. 1A) or left-looking (as in FIG. 1B).

FIG. 2 is a block diagram illustrating an example embodiment of SAR imaging system 200 in accordance with the present systems, devices, methods, and articles. SAR imaging system 200 comprises synthetic aperture radar (SAR) 210 and ground terminal 220. SAR 210 can be mounted on a spaceborne platform such as spaceborne platform 110 a of FIG. 1A, for example a satellite, a spacecraft, or a space station. SAR 210 can be mounted on an airborne platform such as airborne platform 110 b of FIG. 1B, for example an aircraft, an unmanned aircraft, or a drone.

SAR 210 comprises one or more antennas 212, transceiver 214, nontransitory SAR data storage media 216, and SAR data processor 218 (e.g., hardware circuitry). Antenna 212 is bi-directionally communicatively coupled to transceiver 214. Transceiver 214 is bi-directionally communicatively coupled to data storage 216 and data processor 218. Transceiver 214 can include one or more transceiver circuits, for example operable to transmit pulses and receive returned pulses in respective ones of two or more different frequency bands via one or more antenna 212. The transceiver circuits can, for example be commonly housed or on a common circuit board, or housed individually or on respective individual circuit boards. In some implementations, transceiver 214 includes, or consists of, a separate transmitter and receiver, commonly housed or separately housed. Data storage 216 is bi-directionally communicatively coupled to data processor 218.

Data storage 216 can take the form of one or more computer- or processor-readable memories or storage media, for instance volatile memory (e.g., RAM), nonvolatile memory (e.g., ROM, FLASH, EEPROM), or spinning media (e.g., magnetic disk, optical disk) with associated readers and/or writers.

Data processor 218 can comprise one or more data processing elements such as a modulator, an encoder, an encrypter and the like. Data processor 218 can also comprise one or more control elements such as a controller to determine when to switch modes of operation, to command the SAR to switch operation and to synchronize operations in each mode.

Data processor 218 can take the form of one or more circuits or circuitry or hardware, for instance one or more microprocessors (single or multicore), central processor units (CPUs), digital signal processors (DSPs), graphic processing units (GPUs), application specific integrated circuits (ASICs), programmable gate arrays (PGAs), or programmable logic units (PLUs).

Ground terminal 220 comprises antenna 222, transceiver 224, terminal data storage 226, terminal data processor 218, and terminal display 219. Antenna 222 is bi-directionally communicatively coupled to transceiver 224, and transceiver 224 is communicatively coupled to data storage 226. Data received at antenna 222 can be communicated to transceiver 224, and stored in data storage 226. Transceiver 224 can transmit data via antenna 222. In some implementations, transceiver 224 includes, or consists of, a separate transmitter and receiver, commonly housed or separately housed. Data storage 226 is bi-directionally communicatively coupled to data processor 228. Display 229 can receive data for display from data storage 226 and data processor 228.

Data storage 226 can take the form of one or more computer- or processor-readable memories or storage media, for instance volatile memory (e.g., RAM), nonvolatile memory (e.g., ROM, FLASH, EEPROM), or spinning media (e.g., magnetic disk, optical disk) with associated readers and/or writers. Data processor 228 can take the form of one or more circuits or circuitry or hardware, for instance one or more microprocessors (single or multicore), central processor units (CPUs), digital signal processors (DSPs), graphic processing units (GPUs), application specific integrated circuits (ASICs), programmable gate arrays (PGAs), or programmable logic units (PLUs).

In some implementations, SAR 210 has a sufficiently large transmit power that antenna 222 of ground terminal 220 can be relatively small. For example, antenna 222 of ground terminal 220 can be small enough that ground terminal 220 is man-portable, and consequently easily deployed, such as in search and rescue operations. Terminal data processor 218 typically has sufficient processing power to generate SAR images by SAR data processing in near real-time, for example within an hour of reception of the raw SAR data by ground terminal 220.

FIG. 3 illustrates operation of an example embodiment of a SAR imaging system in accordance with the present systems, devices, methods, and articles. The SAR imaging system comprises a SAR and a ground terminal (such as SAR 210 and ground terminal 220 of FIG. 2). The SAR platform (not shown in FIG. 3) overflies a region of interest (enclosed by broken line 310), and beam ground track 320 intersects region of interest 310. Beam ground track 320 enters region of interest 310 at position 312, and leaves region of interest 310 at position 314.

The SAR imaging system transmits interrogation pulses at periodic intervals when the SAR imaging system is at various positions 330A, 330B, 330C, and 330D (collectively 330) in its transit, before and during its overflight of region of interest 310. The interrogation pulses can be generated using a broad-beam mode of the SAR imaging system.

In an example implementation, the SAR transmits pulses at a low pulse repetition frequency in the range 0.5 Hz to 50 Hz (e.g., 20 Hz) when in the broad-beam mode. Each pulse can be an indication to the ground terminal, or to a user on the ground, that the SAR is approaching, and ready to be commanded into an imaging mode by the ground terminal. The SAR antenna beam in the broad-beam mode can be generated, for example, by decreasing the active antenna area, or by beam shaping, either in range or azimuth, or both.

In some implementations, the SAR antenna includes more than one panel. For example, the SAR antenna may include six panels. The SAR antenna beam in the broad-beam mode can be generated by reducing the number of panels used. For example, the SAR antenna beam in broad-beam mode can be generated from a single antenna panel of a multi-panel antenna.

A ground terminal (not shown in FIG. 3) can receive an interrogation pulse transmitted in the broad-beam mode. The ground terminal can respond by firstly acknowledging receipt of the interrogation pulse, and then transmitting a set of coordinates to the SAR defining a desired image ground area 340. In some examples, the set of coordinates include the latitude and the longitude of the center of desired image ground area 340. In some examples, the center of desired image ground area 340 is coincident with the location of the ground terminal. In other examples, the center of desired image ground area 340 is selected such that the ground terminal and the desired image ground area are both within the SAR imaging beam.

In an example implementation, the SAR transmits at L-band (A=24 cm) and has along-track antenna dimension D=6 m. In this example, the broad-beam mode of the SAR transmits a beam having a cross-track beamwidth (defined as the angle of the half-power points of the main lobe) of θCT=20°, and an along-track beamwidth λ/D of θ_(AT)=2.3°. At a range of approximately 450 km, the along-track beamwidth on the ground is approximately 18 km.

The pulses transmitted by the SAR in the broad-beam mode can be narrow-band pulses, selected so that there is sufficient link margin for the ground terminal (such as ground terminal 220 of FIG. 2) to receive the pulses. The ground terminal can receive an interrogation pulse via a sidelobe of the SAR beam, for example the first sidelobe or the second sidelobe of the SAR beam. The first and the second sidelobes of the antenna pattern of the SAR beam are typically approximately 13 dB and 28 dB below the mainlobe, respectively. In an example implementation, the ground terminal receives one of the interrogation pulses via the first sidelobe of the SAR beam, providing approximately 2.5 seconds before the satellite is able to image desired image ground area 340, such as a region centered on the ground terminal's coordinates.

In response to receiving the set of coordinates from the ground terminal, the SAR switches to a narrow-beam mode (the narrow-beam mode having a beamwidth narrower than the broad-beam mode), and starts data transmission and collection for imaging.

FIG. 4 is a timing diagram illustrated an example timing sequence for operation of a SAR imaging system in accordance with the present systems, devices, methods, and articles. FIG. 4 shows three synchronized timing graphs 410, 420, and 430. Graph 410 shows a plot of along-track beamwidth θ_(AT) as a function of time t. Graph 420 shows a plot of cross-track beamwidth θ_(CT) as a function of time t. Graph 430 shows a plot of incidence angle of the beam center θ_(i) as a function of time t. The incidence angle of the beam center θ_(i) is related to the cross-track position of the beam center on the ground.

At time t₁, the SAR imaging system switches to an interrogation mode and starts to transmit interrogation pulses (such as pulses 330 of FIG. 3). In some implementations, along-track beamwidth θ_(AT) is unchanged, and has a value of θ_(AT1) both before and after time t₁. In other implementations, along-track beamwidth θ_(AT) can be broadened and (optionally) steered, and changes value from θ_(AT1) to θ_(AT2) at time t₁.

Cross-track beamwidth θ_(CT) undergoes a stepwise increase as the SAR imaging system changes from a narrow-beam mode to a broad-beam mode. At time t₁, cross-track beamwidth θ_(CT) changes from a value of θ_(CT1) to a value of θ_(CT2).

At time t₁, incidence angle of the beam center θ_(i) can change from a value of θ_(i1) to a value of θ_(i2).

At time t₂, the SAR imaging system receives a response from the ground terminal. The along-track and cross-track beamwidths, and the incidence angle of the beam center remain unchanged.

At time t₃, the SAR imaging system receives the coordinates from the ground terminal and switches to an imaging mode. Along-track beamwidth θ_(AT) may change from θ_(AT2) to θ_(AT3) at time t₃. In some examples, θ_(AT3) can be the same as θ_(AT1).

Cross-track beamwidth θ_(CT) undergoes a stepwise decrease as the SAR imaging system changes from a broad-beam mode to a narrow-beam imaging mode. At time t₃, cross-track beamwidth θ_(CT) changes from a value of θ_(CT2) to a value of θ_(CT3).

At time t₃, incidence angle of the beam center θ_(i) can change from a value of θ_(i2) to a value of θ_(i3) that accommodates the coordinates of the ground terminal and the desired image ground area. Incidence angles of the beam center θ_(i1), θ_(i2), and θ_(i3) are incidence angles within the incidence angle capability of the SAR.

At time t₄, the SAR imaging system begins imaging of the desired image ground area, centered on the coordinates provided by the ground terminal.

In an example implementation, values of along-track beamwidth, cross-track beamwidth, and incidence angle at beam center are as follows: θ_(AT1)=2.3°,θ_(AT2)=2.3° θ_(CT1)=8°,θ_(CT2)=20°,θ_(XT3)=8° θ_(i1)=25°,θ_(i2)=30°,θ_(i3)=35°

The first transmitted pulse in the narrow-beam mode is a linear FM chirp. The ground terminal receives and stores the transmitted pulse for use as a reference function.

The transmitted pulse is backscattered and received at the SAR. The SAR digitizes the received pulse, encodes it, and (optionally) encrypts it. The received data is used as the next transmitted pulse. It is generally noise-like, and has a desirable autocorrelation function.

The ground terminal receives the second transmitted pulse, which is the encrypted encoded return from the first pulse. The ground terminal decrypts the second transmitted pulse, decodes the decrypted second transmitted pulse, and processes the decoded and decrypted second transmitted pulse with the linear FM reference pulse that was captured from the first transmission. The ground terminal now has the first range line of a SAR image, and the reference function for the next range line, where the reference function for the next range line is derived from the second transmitted pulse.

The second transmitted pulse (the encrypted encoded return from the first transmitted pulse) is backscattered and received at the SAR. As before, the SAR digitizes the second transmitted pulse, encodes the second transmitted pulse, and (optionally) encrypts the encoded second transmitted pulse. Then the SAR uses the new “data” as the next transmit pulse. It too is generally noise-like, and has a desirable autocorrelation function.

The ground terminal receives the encrypted encoded return from the second pulse, decrypts the encrypted encoded return, decodes it, and processes it with the encrypted transmission that was captured from the second transmission. The ground terminal now has the second range line as well as the reference function for the next range line derived from the third transmitted pulse. This process is repeated for the range lines needed to form a SAR image.

More detail is provided with reference to FIGS. 5 to 11.

FIG. 5 is a flow chart illustrating a method of operation 500 of a SAR imaging system in accordance with the present systems, devices, methods, and articles. Method 500 begins at 505, for example in response to a request from an operator input or in response to a command from another system.

At 510, the SAR imaging system switches to an interrogation mode, and, at 520, broadcasts an interrogation signal. In some implementations, the interrogation signal can be a sequence of interrogation pulses (such as pulses 330 of FIG. 3). For example, the interrogation signal can be a sequence of pulses generated in a broad-beam mode of the SAR and transmitted at a PRF of 20 Hz.

At 522, the SAR imaging system determines if a response to the interrogation signal from a ground terminal has been received. In response to determining that a response to the interrogation signal has been received (YES) at 522, control in method 500 proceeds to 524. In response to determining that a response to the interrogation signal has not been received (NO) at 522, control in method 500 returns to 520. The loop defined by 520 and 522 causes method 500 to wait until a response from a ground terminal to an interrogation signal from the SAR has been received at the SAR, or until the SAR imaging system exits the interrogation mode, for example upon satisfying a timeout condition.

At 524, the SAR imaging system determines if position information from the corresponding ground terminal, such as the center of the desired image ground area, has been received. In response to determining that position information has been received (YES) at 524, control in method 500 proceeds to 530. In response to determining that position information has not been received (NO) at 524, control in method 500 returns to 524. The loop defined by 524 causes method 500 to wait until position information for the responding ground terminal has been received, or until the SAR imaging system exits the interrogation mode, for example upon satisfying a timeout condition.

At 530, the SAR imaging system switches to an imaging mode and initiates generation of an image. At 535, the SAR imaging system determines the self-image is complete. In response to determining that the image is complete (YES) at 535, control in method 500 proceeds to 540. In response to determining that the image is not complete (NO) at 535, control in method 500 returns to 535. The loop defined by 535 causes method 500 to wait until the self-image is complete, or until the SAR imaging system terminates the generation of the image, for example upon satisfying a timeout condition and/or exiting the imaging mode.

At 540, the SAR imaging system switches to a normal mode of operation or back to interrogation mode, depending on the SAR imaging system's tasking plan, or in response to a request from an operator or a command from another system, and method 500 terminates at 550, for example until called or invoked again.

FIG. 6 is a timing diagram illustrating an example timing sequence 600 for transmitted and received pulses during operation of a SAR imaging system in accordance with the present systems, devices, methods, and articles. Timing sequence 600 comprises transmit (TX) timing sequence 610, and receive (RX) timing sequence 620.

FIG. 6 illustrates a sequence of three transmitted pulses 612, 614, and 616, and three corresponding received pulses 622, 624, and 626. In operation, the SAR imaging system can transmit (TX) and receive (RX) any suitable number of pulses. In some implementations, one or more of the transmitted pulses 612, 614, 616, etc. are linear frequency-modulated (FM) pulses. Each received pulse (622, 624, 626, etc.) is a superposition of reflections of a corresponding transmitted pulse (612, 614, 616, etc. respectively). The reflections are from targets on the ground. Received pulses 622, 624, 626, etc. are typically noise-like.

FIG. 7 is a flow chart illustrating an example method 700 of encoding a transmitted pulse of a SAR imaging system in accordance with the present systems, devices, methods, and articles. Method 700 begins at 705 for example in response to a request from an operator or in response to a command from another system.

At 710, the SAR imaging system transmits a first linear FM pulse (such as transmitted pulse 612 of FIG. 6), and, at 720, receives the first pulse (such as received pulse 622 of FIG. 6) reflected from the ground.

At 730, the SAR imaging system samples the first received pulse, for example to generate 8-bit in-phase (I) and quadrature (Q) samples, and, at 735, generates an uncompressed range line. The uncompressed range line can typically be a quasi-stationary Gaussian signal, for example where the pulse has been backscattered from a distributed target.

In a first encoding operation, at 740, the SAR imaging system encodes the data, for example to 8PSK (Phase-Shift Keying using 8 phases). PSK is a digital modulation technique in which a base signal is phase-modulated to represent the data. In a second encoding operation, at 745, the SAR imaging system convolutionally encodes and (optionally) encrypts the 8PSK-encoded data. Other encoding schemes can be used.

For example, 16PSK modulation can be used. The selected encoding scheme can depend, for example, on the size of the power of the SAR antenna and the antenna gain-to-noise-temperature (G/T) of the ground terminal antenna. A 16PSK scheme may be more suitable for a high power SAR transmitter and/or a high ground terminal antenna G/T. An 8PSK scheme may be more suitable for a lower power SAR transmitter and/or a lower ground terminal antenna G/T. If the transmitter has amplitude modulation capability, then the transmitted pulse can be encoded, at least in part, using amplitude modulation. Amplitude and phase-shift keying (APSK) can be used as a modulation scheme. In an example scheme, 8PSK is augmented by four additional vectors at half-amplitude at phase angles of 0°, 90°, 180°, and 270°.

The goal of encoding the data is usually to achieve a uniform power spectral density within the SAR operating bandwidth. A uniform power spectral density is typically associated with pseudo-random noise, which is desirable for the encoded data because the autocorrelation function (also known as the range point spread function) will be close to an ideal sin(x)/x curve for pseudo-random noise.

At 750, the SAR imaging system transmits the encoded first received pulse as a second transmitted pulse. Method 700 terminates at 760, for example until called or invoked again.

FIG. 8 is a flow chart illustrating another example method 800 of encoding a transmitted pulse of a SAR imaging system in accordance with the present systems, devices, methods, and articles. Method 800 begins at 805 for example in response to a request from an operator or in response to a command from another system.

At 810, the SAR imaging system transmits the n^(th) pulse, for example using PR 8PSK encoding as described above, and, at 820, receives the n^(th) pulse reflected from the ground.

At 830, the SAR imaging system samples the n^(th) received pulse, for example to generate 8-bit in-phase (I) and quadrature (Q) samples, and, at 835, generates an uncompressed range line.

In a first encoding operation, at 840, the SAR imaging system encodes the data, for example to 8PSK (Phase-Shift Keying using 8 phases).

PSK is a digital modulation technique in which a base signal is phase-modulated to represent the data. In a second encoding operation, at 845, the SAR imaging system convolutionally encodes and (optionally) encrypts the 8PSK-encoded data.

At 850, the SAR imaging system transmits the encoded n^(th) received pulse as the n+1^(th) transmitted pulse. Method 800 terminates at 860, for example until called or invoked again.

FIG. 9 is a flow chart illustrating a method 900 of range compressing received pulses by a ground terminal in accordance with the present systems, devices, methods, and articles.

At 910, the ground terminal retrieves a copy of TX1, the first transmitted pulse. In some implementations, as described above, TX1 is a linear FM pulse. At 920, the ground terminal retrieves a copy of TX2, the second transmitted pulse built from the first received pulse RX1 (see for example FIG. 6).

At 930 and 935, the ground terminal decodes and demodulates TX2 in a first and a second act, respectively. The first act can include decryption and decoding, and the second act can include demodulation. At 940, the ground terminal regenerates RX1, the first received pulse, and, at 950, the ground terminal performs range compression using TX1 and RX1, the replica and the first received pulse, respectively.

At 960, the ground terminal stores the first range line (RX1 range-compressed).

FIG. 10 is a flow chart illustrating another method 1000 of range compressing received pulses by a ground terminal in accordance with the present systems, devices, methods, and articles.

At 1010, the ground terminal retrieves a copy of TX2, the first transmitted pulse. In some implementations, as described above, TX2 is a transmitted pulse built from RX1. At 1020, the ground terminal retrieves a copy of TX3, the third transmitted pulse built from the second received pulse RX2 (see for example FIG. 6).

At 1030 and 1035, the ground terminal decodes TX3 in a first and a second act, respectively. At 1040, the ground terminal regenerates RX2, the second received pulse, and, at 1050, the ground terminal performs range compression using TX2 and RX2, the replica and the second received pulse, respectively.

At 1060, the ground terminal stores the second range line (RX2 range-compressed).

FIG. 11 is a flow chart illustrating an example method 1100 of forming a self-image by a ground terminal in accordance with the present systems, devices, methods, and articles. The flow chart in FIG. 11 describes acts performed by a data processor such as terminal data processor 280 of FIG. 2.

At 1110, the data processor performs range compression on the first received pulse (RX1). At 1120, the data processor performs range compression on the next received pulse. At 1125, the data processor determines whether there are more range lines to compress. In response to determining there are more range lines to compress, control of method 1100 returns to 1120. In response to determining there are no more range lines to compress, control of method 1100 proceed to 1130. The loop defined by 1120 and 1125 causes method 1100 to range compress lines needed to form a desired SAR image from the data.

At 1130, the data processor optionally performs range cell migration correction (as indicated by the dashed box). Range cell migration correction may be required depending on the azimuth beamwidth and the range resolution. At 1140, the data processor performs azimuth compression using the range compressed (and optionally range cell migration corrected) data.

At 1150, the data processor outputs a SAR image for storage, transfer over a network, and/or display on the ground terminal.

Method 1100 is usually referred to as a range-Doppler method for generating SAR images. Other approaches can be used that also include building a transmitted pulse from a backscattered and previously received pulse. Range cell migration correction can be included, as described above, as required by the azimuth beamwidth and the range resolution.

In some radars, such as airborne synthetic aperture radars, an echo of a transmitted pulse (e.g., the transmitted pulse backscattered by a target such as the ground) is received directly after the transmitted pulse, and before the next pulse is transmitted. In other radars, such as spaceborne SARs, an echo of the transmitted pulse is received after one or more subsequent pulses have been transmitted. The intervening pulses are in flight between the radar and the target—on their way to the target or on their way back from the target. The number of intervening pulses can depend on the viewing geometry of the radar.

When there are intervening pulses in flight between a transmitted pulse and its received echo, the systems and methods in the present disclosure describe the transmission of an n^(th) pulse, the encoding of the echo of the n^(th) pulse, and the transmission of the encoded echo of the n^(th) pulse as the m^(th) transmitted pulse, where m>n. If there are no intervening pulses, then m=n+1.

In some implementations, the radar is a pulse radar. In other implementations, the radar is a SAR.

FIG. 12 is a flow chart illustrating an example method 1200 of encoding a transmitted pulse of a radar in accordance with the present systems, devices, methods, and articles. Method 1200 begins at 1205 for example in response to a request from an operator or in response to a command from another system.

At 1210, the radar transmits an n^(th) pulse in a sequence of N pulses. The n^(th) pulse may be a linear FM pulse (such as transmitted pulse 612 of FIG. 6), or another suitable pulse such as an unmodulated pulse or a pulse that includes encoded information. The encoded information can include a received echo from a previously transmitted pulse, for example.

At 1220, the radar receives the n^(th) pulse reflected from a target. The target can be a point target or a distributed target. The radar transmission can be directed at the ground from an airborne or spaceborne vehicle, for example, and the target can be the ground.

At 1230, the radar samples the received pulse, for example to generate in-phase (I) and quadrature (Q) samples. The I and Q samples can be 8-bit samples, for example. In a synthetic aperture radar, the samples can be used to generate an uncompressed range line. The uncompressed range line can be a quasi-stationary Gaussian signal, for example where the pulse has been backscattered from a distributed target.

The sampled pulse can be encoded. Encoding can be performed in a single operation or in multiple operations. In the example illustrated in the FIG. 12, encoding is performed by two operations. In a first encoding operation, at 1240, the radar encodes the data, for example to 8PSK (Phase-Shift Keying using 8 phases). PSK is a digital modulation technique in which a base signal is phase-modulated to represent the data. In a second encoding operation, at 1245, the radar convolutionally encodes the data output from the first encoding operation. At 1250, the radar (optionally) encrypts the encoded data output from the second encoding operation.

Other suitable encoding and/or modulation schemes can be used. For example, 16PSK modulation can be used. The selected encoding scheme can depend, for example, on the size of the power of the radar antenna and the antenna gain-to-noise-temperature (G/T) of a ground terminal antenna used to receive the transmitted pulses. In the case of a synthetic aperture radar, a 16PSK scheme may be more suitable for a high power SAR transmitter and/or a high ground terminal antenna G/T. An 8PSK scheme may be more suitable for a lower power SAR transmitter and/or a lower ground terminal antenna G/T. If the transmitter has amplitude modulation capability, then the transmitted pulse can be encoded, at least in part, using amplitude modulation. Amplitude and phase-shift keying (APSK) can be used as a modulation scheme. In an example scheme, 8PSK is augmented by four additional vectors at half-amplitude at phase angles of 0°, 90°, 180°, and 270°.

The goal of encoding the data is usually to achieve a uniform power spectral density within the radar's operating bandwidth. A uniform power spectral density is typically associated with pseudo-random noise, which is desirable for the encoded data because the autocorrelation function (also known as the range point spread function) will be close to an ideal sin(x)/x curve for pseudo-random noise.

At 1260, the radar generates an m^(th) transmitted pulse based at least in part on the encoded, and optionally encrypted, received n^(th) pulse. At 1270, the radar transmits the m^(th) transmitted pulse.

Method 1200 terminates at 1280, for example until called or invoked again.

FIG. 13 is a flow chart illustrating a method 1300 of range-compressing received pulses by a ground terminal in accordance with the present systems, devices, methods, and articles.

At 1310, the ground terminal retrieves a copy of TXn, the n^(th) transmitted pulse. At 1320, the ground terminal retrieves a copy of TXm, the m^(th) transmitted pulse built from the n^(th) received pulse RXn.

In some implementations, TXm is encrypted, and, at 1325, the ground terminal decrypts TXm. Dashed lines are used in 1325 of FIG. 13 to indicate that decryption is optional. Decoding of TXm can be performed in one or more acts. In the illustrated example of FIG. 13, the ground terminal decodes and/or demodulates TXm in single act 1330. In one example implementation, the first act can include decryption and decoding, and the second act can include demodulation. At 1340, the ground terminal regenerates RXn, the n^(th) received pulse, and, at 1350, the ground terminal performs range compression using TXn and RXn, the replica and the n^(th) received pulse, respectively.

At 1360, the ground terminal stores a compressed range line (RXn range-compressed).

While the foregoing description refers, for the most part, to satellite platforms for SAR and optical sensors, remotely sensed imagery can be acquired using airborne sensors including, but not limited to, aircraft, unmanned aircraft, and drones. The technology described in this disclosure can, for example, be applied to imagery acquired from sensors on spaceborne platforms and/or airborne platforms.

A single frequency SAR can generate images of the ground (e.g., land, water, ice, snow, targets or objects) by transmitting radar pulses in a frequency band centered on a single frequency. Multi-band SAR can be used to acquire SAR images at different frequency bands at the same time. Simultaneous, or near-simultaneous acquisition of SAR images at more than one frequency band (for example, at L-band and X-band) can provide a more complete understanding of the ground than acquisition of SAR images at a single band. In some implementations, acquisition of SAR images at more than one frequency band can at least partially overlap in time, or can be within seconds of each other, within the same pass, or within the same acquisition window. Acquired multi-band SAR images can overlap in geographic coverage. With multi-band SAR, data acquired at each of the different frequency bands can be single-polarization or multi-polarization SAR data.

An X-band SAR typically operates at frequencies in the range of 8 GHz to 12 GHz. An L-band SAR typically operates at frequencies in the range of 1 GHz to 2 GHz.

Some multi-band SAR systems, operate at more than one frequency band using separate apertures. Others can operate using a shared aperture. Phased array antennas can comprise an array of constituent antennas or radiating elements. Each radiating element can be fed by a signal whose phase and amplitude, relative to the phase and amplitude of the signal fed to the other radiating elements, can be adjusted so as to generate a desired radiation pattern for the phased array antenna. Phased array antennas are described, for example in published PCT International Application No. PCT/US2016/037666 (International Publication No. WO/2017/044168). In implementations of SAR imaging systems in accordance with the present systems, devices, methods, and articles, where the SAR is a dual-band or multi-band SAR, pulses are transmitted in two or more frequency bands, and pulses transmitted in at least one of the frequency bands can include encoded returns from the backscattering of transmitted pulses in the same frequency band as the transmitted pulse or in another frequency band.

For example, in some embodiments, using a dual-band SAR, data is collected at a first frequency band and at a second frequency band, for example concurrently. Data from the backscattered energy of the pulses transmitted at first frequency band and the second frequency band is encoded, and the encoded data from both frequency bands is transmitted in subsequent pulses of at least the first frequency band. Thus, data can be collected at two or more frequency bands via one set of pulses, encoded into and transmitted together in one or more subsequent pulses, for instance, encoded in a subsequent pulse of one of the frequency bands. As well as being used to collect subsequent data, subsequent pulses can also concurrently transmit previously collected data to the ground in a same pulse that is being used to collect subsequent data.

In some embodiments using a dual-band SAR, data is collected at second frequency band (and optionally a first frequency band), and data from backscattered energy of the pulse transmitted at the second frequency band is encoded, and the encoded data from only the second frequency band is transmitted from the SAR platform in subsequent pulses at the first frequency band.

The data collected via a returned pulse or a set of returned pulses can be encoded in a very next pulse to be transmitted after either transmission or return of the pulse or set of pulses which collected the instance of data. Alternatively, in some implementations, the data collected via a returned pulse or a set of returned pulses can be encoded in a subsequent pulse to be transmitted after either transmission or return of the pulse or set of pulses which collected the instance of data, with one or more intervening pulses being transmitted between the pulse or set of pulses which resulted in the collection of a given instance of collected data, and a subsequent pulse that encodes the instance of collected data. Those intervening pulses may not encode any data, or may encode data collected via returns of one or more previously transmitted pulses or sets of pulses. For instance, an n^(th) instance of data collected via an n^(th) pulse or set of pulses to be transmitted may be encoded in a pulse of an n^(th)+i pulse or set of pulses to be transmitted, where i is an integer equal to or greater than 1. Encoding data may include modulating collected data to generate modulated data, and convolutionally encoding modulated data to generate encoded data.

FIG. 14 is a schematic diagram illustrating an illumination geometry of an example embodiment of a dual-band SAR imaging system in accordance with the present systems, devices, methods, and articles. The SAR imaging system of FIG. 14 comprises a spaceborne platform 1410 and a ground terminal 1420. Spaceborne platform 1410 comprises a dual-band synthetic aperture radar (not shown in FIG. 14). In some implementations, dual-band SAR is a dual-band XL SAR that is operable at X-band and L-band. Elements of a synthetic aperture radar (SAR) are described in reference to FIG. 2 (above). In some implementations, antenna 212 of FIG. 2 is a dual-band or multi-band phased array antenna.

Spaceborne platform 1410 can be, for example, a satellite, a spacecraft, or a space station. In some implementations, spaceborne platform 1410 can be replaced by an aircraft or an unmanned aircraft such as a drone. In some implementations, ground terminal 1420 is mobile. In other implementations, ground terminal 1420 is fixed. In some implementations, ground terminal 1420 is man-portable. Ground terminal 1420 can be located on a surface of the Earth, for example, on land, water, or ice. Ground terminal 1420 can also be located in the air. For example, ground terminal 1420 could be located on a ship, submarine, floating platform, buoy or other water-borne vessel or structure; on an aircraft, drone, balloon or other airborne vessel or structure; or on a land vehicle, building or other terrestrial vessel or structure.

In the illustrated implementation, spaceborne platform 1410 flies along trajectory 1430. Dashed line 1440 indicates the ground track of spaceborne platform 1410. Shaded region 1460 a represents a main lobe of a SAR antenna beam pattern on the ground for a first frequency band (for example, L-band). Line 1450 and line 1455 indicate a near-side and a far-side, respectively, of a swath at the first frequency band.

Shaded region 1460 b represents a main lobe of a SAR antenna beam pattern on the ground for a second frequency band (for example, X-band). In FIG. 14, shaded region 1460 b is shown as having a smaller illuminated area and narrower swath width than shaded region 1460 a. However, the illuminated area and swath widths of the first and second frequency beams can be the same or can be different. The illuminated areas 1460 a and 1460 b can partially overlap (as shown) or not, or one can encompass the other. In some implementations, at least one of the SAR antenna beams is squinted, i.e., directed to the ground at an angle forward or aft of broadside to the track of the SAR antenna.

In the configuration illustrated in FIG. 14, as spaceborne platform 1410 flies along trajectory 1430, ground terminal 1420 will first enter and then leave the second frequency band SAR antenna beam, as represented by shaded region 1460 b, and then enter and leave the first frequency band SAR antenna beam, as represented by shaded region 1460 a. The SAR antenna beams at the two different frequencies can be pointing in the same general direction (as shown in FIG. 14) or can be pointing in different directions. In some configurations and trajectories, ground terminal 1420 will be illuminated by only one of the beams.

FIG. 15 is a flow chart illustrating an example method 1500 of encoding a pulse transmitted from a dual-band radar at a first frequency band with returns received from the transmission of pulses at a first and a second frequency band. Method 1500 begins at 1505 for example in response to a request from an operator or in response to a command from another system.

At 1510 a, the radar transmits an n^(th) pulse, at a first frequency band, in a sequence of N pulses. At 1510 b, the radar transmits an n^(th) pulse, at a second frequency band, in a sequence of N pulses. The n^(th) pulses at the first and second frequency bands can be transmitted simultaneously, or the relative timing of their transmission can be synchronized or coordinated as desired. Each pulse may be a respective linear FM pulse, or other suitable pulse such as an unmodulated pulse or a pulse that includes encoded information. The encoded information can include a received return from one or more previously transmitted pulses, for example. The radar transmission can be directed to the ground from an airborne or spaceborne vehicle or platform, for example. Directing the radar transmission to the ground, and receiving backscattered energy, can include directing the radar transmission to targets on the ground, and receiving backscattered energy from the targets. Targets may include man-made structures, vehicles, and the like.

At 1520 a, the radar receives the n^(th) first-frequency pulse backscattered from the ground, for example, from a target or region on the Earth's surface, for example, on land, water, snow or ice. At 1520 b, the radar receives the n^(th) second-frequency pulse backscattered from the ground.

At 1530 a, the radar samples the received first-frequency pulse, backscattered from the ground. At 1530 b, the radar samples the received second-frequency pulse, backscattered from the ground. For example, the radar may generate in-phase (I) and quadrature (Q) samples from each received pulse. The I and Q samples can be 8-bit samples, for example. In a synthetic aperture radar, the samples can be used to generate an uncompressed range line. The uncompressed range line can be a quasi-stationary Gaussian signal, for example where the pulse has been backscattered from a distributed target.

The sampled pulses can be encoded. Encoding can be performed in a single operation or in multiple operations. In the example illustrated in the FIG. 15, encoding is performed by two operations. In a first encoding operation, at 1540, the radar or other suitable encoder circuitry encodes the data, for example to 8PSK (Phase-Shift Keying using 8 phases). PSK is a digital modulation technique in which a base signal is phase-modulated to represent the data. In a second encoding operation, at 1545, the radar or other suitable encoder circuitry convolutionally encodes the data output from the first encoding operation. Optionally at 1550, the radar, or other suitable encryption circuitry, encrypts the encoded data output from the second encoding operation.

Other suitable encoding and/or modulation schemes can be used. For example, 16PSK modulation can be used. The selected encoding scheme can depend, for example, on the size of the power of the radar antenna and the antenna gain-to-noise-temperature (G/T) of a ground terminal antenna used to receive the transmitted pulses. In the case of a synthetic aperture radar, a 16PSK scheme may be more suitable for a high power SAR transmitter and/or a high ground terminal antenna G/T. An 8PSK scheme may be more suitable for a lower power SAR transmitter and/or a lower ground terminal antenna G/T. If the transmitter has amplitude modulation capability, then the transmitted pulse can be encoded, at least in part, using amplitude modulation. Amplitude and phase-shift keying (APSK) can be used as a modulation scheme. In an example scheme, 8PSK is augmented by four additional vectors at half-amplitude at phase angles of 0°, 90°, 180°, and 270°.

The goal of encoding the data is usually to achieve an approximately uniform power spectral density within the radar's operating bandwidth. A uniform power spectral density is typically associated with pseudo-random noise, which is desirable for the encoded data because the autocorrelation function (also known as the range point spread function) will be close to an ideal sin(x)/x curve for pseudo-random noise.

At 1560, the radar generates an m^(th) transmitted pulse at a first frequency band based at least in part on the encoded, and optionally encrypted, received n^(th) pulses at each of the first and the second frequency bands. At 1570, the radar transmits an m^(th) transmitted pulse at the first frequency band. The m^(th) transmitted pulse sent at the first frequency band contains information from pulses previously transmitted and received at both the first and the second frequency bands.

Method 1500 terminates at 1580, for example until called or invoked again, or can be repeated for a plurality of subsequent pulses.

One example use case of the method illustrated in FIG. 15 is as follows. It can be beneficial for a search and rescue unit (e.g., on land or on water or snow or ice) to command acquisition of SAR data from a dual-band SAR, receive the data at a ground terminal co-located with the search and rescue unit, and process the data to form a SAR image at each of the two bands. One of the antenna beams may be broader than the other, and can be used to transmit the encoded radar returns to the ground terminal, whether or not the ground terminal is within the beamwidth of the second (narrower) beam.

Images in two bands can provide complementary information about the scene being imaged. Use of images in two bands can improve classification, for example. Images in two bands can also be fused to provide an improved picture of the scene for a user of the data such as the search and rescue unit in this case.

FIG. 16 is a flow chart illustrating an example method 1600 of encoding a pulse transmitted from a dual-band radar at a first frequency band with returns received from the transmission of a pulse at a second frequency band. Method 1600 begins at 1605 for example in response to a request from an operator or in response to a command from another system.

At 1610 a, the radar transmits an n^(th) pulse, at a first frequency band, in a sequence of N pulses. At 1610 b, the radar transmits an n^(th) pulse, at a second frequency band, in a sequence of N pulses. The n^(th) pulses at the first and second frequency bands can be transmitted simultaneously, or the relative timing of their transmission can be synchronized or coordinated as desired. Each pulse may be a respective linear FM pulse, or other suitable pulse such as an unmodulated pulse or a pulse that includes encoded information. The encoded information can include a received return from one or more previously transmitted pulses, for example. The radar transmission can be directed to the ground from an airborne or spaceborne vehicle or platform, for example.

At 1620, the radar receives the n^(th) second-frequency pulse backscattered from the ground, for example, from a target or region on the Earth's surface, for example, on land, water, snow or ice.

At 1630, the radar samples the received second-frequency pulse. For example, the radar may generate in-phase (I) and quadrature (Q) samples from a received pulse. The I and Q samples can be 8-bit samples, for example. In a synthetic aperture radar, the samples can be used to generate an uncompressed range line. The uncompressed range line can be a quasi-stationary Gaussian signal, for example where the pulse has been backscattered from a distributed target.

The sampled pulse can be encoded. Encoding can be performed in a single operation or in multiple operations. In the example illustrated in the FIG. 16, encoding is performed by two operations. In a first encoding operation, at 1640, the radar or other suitable encoder circuitry encodes the data, for example to 8PSK (Phase-Shift Keying using 8 phases). PSK is a digital modulation technique in which a base signal is phase-modulated to represent the data. In a second encoding operation, at 1645, the radar or other suitable encoder circuitry convolutionally encodes the data output from the first encoding operation. Optionally at 1650, the radar or other suitable encryption circuitry encrypts the encoded data output from the second encoding operation.

At 1660, the radar generates an m^(th) transmitted pulse at a first frequency band based at least in part on the encoded, and optionally encrypted, received n^(th) pulse at the second frequency band. At 1670, the radar transmits an m^(th) transmitted pulse at the first frequency band. The m^(th) transmitted pulse sent at the first frequency band contains information from the illumination by the other beam.

Method 1600 terminates at 1680, for example until called or invoked again, or can be repeated for a plurality of subsequent pulses.

When the ground terminal is not in (e.g., illuminated by) the second beam, the first transmitted pulse in the second band can be encoded in the first transmitted pulse in the first band. In this way, the ground terminal receives a reference function to use in range compression of the data in the second band. Alternatively, the first transmitted pulse in the second band can be received at the ground terminal by another suitable method.

The acquisition of SAR data in the second band can use the same scheme for transmitted pulses as the first band as described above i.e., the SAR encodes each received pulse in a subsequent transmitted pulse in the second band. This is in addition to encoding each received pulse in a subsequent transmitted pulse in the first band.

An example use case of the method illustrated in FIG. 16 is as follows. It can be beneficial for a search and rescue unit (e.g., on land or on water, on snow or ice) to command acquisition of SAR data at a second band from a dual-band SAR, receive the data via a first band at a ground terminal co-located with the search and rescue unit, and process the data to form a SAR image of a scene at the second band. It can be particularly beneficial when the ground terminal is not located within the beamwidth of the second band during imaging. One of the antenna beams may be broader than the other (e.g., the first band), and can be used to transmit to the ground terminal the encoded radar returns for pulses transmitted in the second band.

The foregoing description describes, for the most part, that a pulse transmitted at at least one of the frequency bands includes an encoded return from the backscattering of a single transmitted pulse at another frequency band, or from the backscattering of a single transmitted pulse in the same frequency band and a single transmitted pulse in one or more other frequency bands. In other example implementations, a pulse transmitted at at least one of the frequency bands can include encoded returns from the backscattering of multiple transmitted pulses at another frequency band, or from the backscattering of multiple transmitted pulses at the same frequency band and/or at one or more other frequency bands. Similarly, with a radar operating at a single frequency band, a subsequent transmitted pulse can include encoded returns from the backscattering of more than one previously transmitted pulse.

The various embodiments described above can be combined to provide further embodiments. U.S. Provisional Patent Application Ser. No. 62/260,063, filed Nov. 25, 2015; International Patent Application Serial No. PCT/US2016/063630, filed Nov. 23, 2016; and U.S. Provisional Patent Application Ser. No. 62/510,123, filed May 23, 2017 are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.

As used herein and in the claims, the term transceiver includes one or more transceiver circuits (e.g., an assembly of transceiver circuits), for example operable to transmit pulses and receive returned pulses in respective ones of two or more different frequency bands via one or more antenna. The transceiver circuits can, for example be commonly housed or reside on a common circuit board, or the transceiver circuits can be individually housed or on reside on respective individual circuit boards.

Reference to a first frequency band and a second frequency band typically includes two different frequency bands, which may be either partially overlapping frequency bands, or which may be mutually exclusive, non-overlapping frequency bands.

As used herein and in the claims, the term ground means any one or more of land, water, ice, or snow, or targets located on the ground or in close proximity to the ground.

In some implementations, the SAR platform and ground terminal may each include one or more communications antennas and communications transceivers that are dedicated to communications and control, and which are not used to transmit or receive imaging pulses. Such may, for example, be used to send commands that cause the SAR platform to enter into, and out of, an imaging mode of operation.

The foregoing detailed description has, for instance, set forth various embodiments of the devices and/or processes via the use of block diagrams, schematics, and examples. Insofar as such block diagrams, schematics, and examples contain one or more functions and/or operations, it will be understood by those skilled in the art that each function and/or operation within such block diagrams, flowcharts, or examples can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof. In one embodiment, the present subject matter may be implemented via Application Specific Integrated Circuits (ASICs). However, those skilled in the art will recognize that the embodiments disclosed herein, in whole or in part, can be equivalently implemented in standard integrated circuits, as one or more computer programs running on one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs running on one or more controllers (e.g., microcontrollers) as one or more programs running on one or more processors (e.g., microprocessors), as firmware, or as virtually any combination thereof, and that designing the circuitry and/or writing the code for the software and or firmware would be well within the skill of one of ordinary skill in the art in light of this disclosure.

In addition, those skilled in the art will appreciate that the mechanisms of taught herein are capable of being distributed as a program product in a variety of forms, and that an illustrative embodiment applies equally regardless of the particular type of signal bearing media used to actually carry out the distribution. Examples of signal bearing media include, but are not limited to, the following: recordable type media such as floppy disks, hard disk drives, CD ROMs, digital tape, and computer memory; and transmission type media such as digital and analog communication links using TDM or IP based communication links (e.g., packet links).

These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure. 

What is claimed is:
 1. A method of operation of a multi-band synthetic aperture radar (SAR) comprising a transceiver and a hardware data processor, the method comprising: causing by the data processor the SAR to enter an interrogation mode, and while in the interrogation mode: transmitting by the transceiver one or more interrogation pulses to the ground at a first or a second frequency band, the second frequency band different from the first frequency band; determining by the data processor if a response to the one or more interrogation pulses has been received from a ground terminal; determining by the data processor if position information specifying a ground location has been received from the ground terminal; upon determining by the data processor a response to the one or more interrogation pulses has been received from the ground terminal, and position information specifying a ground location has been received from the ground terminal, causing by the data processor the SAR to enter an imaging mode, and while in the imaging mode: transmitting by the transceiver a first transmitted imaging pulse to the ground at the first frequency band; transmitting by the transceiver a second transmitted imaging pulse to the ground at the second frequency band; receiving by the transceiver a first received pulse that includes backscattered energy from the first transmitted imaging pulse; receiving by the transceiver a second received pulse that includes backscattered energy from the second transmitted imaging pulse; encoding by the data processor data from the first and the second received pulses to generate a first subsequent transmitted imaging pulse; and transmitting by the transceiver the first subsequent transmitted imaging pulse to the ground at the first frequency band.
 2. The method of claim 1 wherein encoding by the data processor data from the first and the second received pulses to generate a subsequent transmitted pulse includes: modulating by the data processor a combination of data from the first and the second received pulses by at least one of phase-shift keying (PSK) or amplitude and phase-shift keying (APSK) to generate a modulated pulse; convolutionally encoding by the data processor the modulated pulse to generate an encoded pulse.
 3. The method of claim 2 wherein encoding by the data processor data from the first and the second received pulses to generate a subsequent transmitted pulse further includes encrypting by the data processor the encoded pulse to generate an encrypted subsequent transmitted pulse.
 4. The method of claim 1 wherein transmitting by the transceiver one or more interrogation pulses to the ground at a first or a second frequency band includes transmitting by the transceiver one or more pulses in a broad-beam mode of the multi-band SAR.
 5. The method of claim 4 wherein transmitting by the transceiver one or more interrogation pulses in a broad-beam mode of the multi-band SAR includes transmitting by the transceiver one or more pulses at a pulse repetition frequency in a range of 0.5 Hz to 50 Hz.
 6. The method of claim 1, further comprising: transmitting by the transceiver a second subsequent transmitted pulse to the ground at the second frequency band; receiving by the transceiver a first subsequent received pulse at the first frequency band that includes backscattered energy from the first subsequent transmitted pulse; receiving by the transceiver a second subsequent received pulse at the second frequency band that includes backscattered energy from the second subsequent transmitted pulse; encoding by the data processor data from the first and the second subsequent received pulses to generate a further subsequent transmitted pulse; and transmitting by the transceiver the further subsequent transmitted pulse to the ground at the first frequency band.
 7. The method of claim 6, further comprising: transmitting by the transceiver a third transmitted pulse to the ground at the first frequency band before transmitting by the transceiver the first subsequent transmitted pulse to the ground at the first frequency band, wherein the transmitting of the first subsequent transmitted pulse to the ground at the first frequency band does not consecutively follow the transmitting of the first transmitted pulse to the ground at the first frequency band.
 8. The method of claim 1 wherein the multi-band SAR is a dual-band SAR, and the first frequency band is L-band and the second frequency band is X-band.
 9. The method of claim 1 wherein the transmitting of pulses at the first frequency band is synchronized with the transmitting of pulses at the second frequency band.
 10. The method of claim 1 wherein the transmitting of pulses at the first frequency band is at a pulse repetition frequency that is substantially the same as for the transmitting of pulses at the second frequency band.
 11. The method of claim 10 wherein the transmitting of pulses at the first frequency band is interleaved with the transmitting of pulses at the second frequency band.
 12. A method of generating synthetic aperture radar (SAR) images by a ground terminal, the ground terminal comprising a transceiver, a data store, and a hardware data processor, the method comprising: receiving by the transceiver an interrogation pulse transmitted by a multi-band SAR at a first or a second frequency band, the second frequency band different from the first frequency band; transmitting by the transceiver a response to the interrogation pulse; transmitting by the transceiver position information specifying a ground location; receiving by the transceiver an n^(th) transmitted first-frequency pulse transmitted by the multi-band SAR at the first frequency band; storing the n^(th) transmitted first-frequency pulse in the data store; receiving by the transceiver an m^(th) transmitted pulse transmitted by the multi-band SAR at the first frequency band, wherein m is greater than n, and wherein a radar return received by the multi-band SAR includes backscattered energy from the n^(th) transmitted first-frequency pulse and backscattered energy from an n^(th) transmitted second-frequency pulse transmitted by the multi-band SAR at the second frequency band, and the m^(th) transmitted pulse transmitted by the multi-band SAR at the first frequency band includes an encoding of the radar return received by the multi-band SAR; storing the m^(th) transmitted pulse in the data store; decoding by the data processor the m^(th) transmitted pulse to regenerate the radar return received by the multi-band SAR; and generating a range line by the data processor for a SAR image by range compression of at least a portion of the radar return received by the SAR using a reference function.
 13. The method of claim 12 wherein the generating a range line by the data processor for a SAR image comprises using the n^(th) transmitted first-frequency pulse as a reference function.
 14. The method of claim 12 wherein the generating a range line by the data processor for a SAR image comprises using a previously transmitted second-frequency pulse, transmitted by the multi-band SAR at the second frequency band, as a reference function.
 15. The method of claim 14, further comprising receiving the previously transmitted second-frequency pulse by the ground terminal.
 16. The method of claim 14 wherein transmitting by the transceiver position information specifying a ground location includes transmitting by the transceiver position information that includes a location of the ground terminal.
 17. The method of claim 14 wherein transmitting by the transceiver position information specifying a location to be imaged includes transmitting by the transceiver position information that includes the center of a desired image ground area.
 18. The method of claim 12 wherein decoding by the data processor the m^(th) transmitted pulse to regenerate the radar return received by the multi-band SAR includes: convolutionally decoding by the data processor the m^(th) transmitted pulse to generate a modulated radar return; and demodulating by the data processor the modulated radar return to regenerate the radar return received by the multi-band SAR.
 19. The method of claim 12 wherein the encoding of the m^(th) transmitted pulse includes an encryption of the radar return received by the multi-band SAR, and wherein decoding the m^(th) transmitted pulse by the data processor to regenerate the radar return received by the multi-band SAR includes: decrypting by the data processor the m^(th) transmitted pulse to generate an encoded radar return; convolutionally decoding by the data processor the encoded radar return to generate a modulated radar return; and demodulating by the data processor the modulated radar return to regenerate the radar return received by the SAR.
 20. The method of claim 12 wherein the multi-band SAR is a dual-band SAR, and the first frequency band is L-band and the second frequency band is X-band.
 21. A synthetic aperture radar (SAR) imaging system, comprising: a multi-band SAR mounted on a SAR platform, the multi-band SAR comprising a SAR antenna that transmits a first and a subsequent transmitted pulse to the ground at a first frequency band, and receives a first received pulse that includes backscattered energy from the first transmitted pulse, and transmits a second transmitted pulse to the ground at a second frequency band, and receives a second received pulse that includes backscattered energy from the second transmitted pulse, the second frequency band different from the first frequency band; a SAR transceiver communicatively coupled to the SAR antenna, the SAR transceiver communicatively coupled to at least one of a SAR data store and a SAR data processor, the SAR data store communicatively coupled to the SAR data processor, the SAR data store which stores the first and the second received pulses, and the SAR data processor which encodes data from the second received pulse to generate the subsequent transmitted pulse for transmission by the SAR antenna to the ground at the first frequency band.
 22. The synthetic aperture radar (SAR) imaging system of claim 21 wherein the SAR data processor encodes data from the first and the second received pulse to generate the subsequent transmitted pulse for transmission by the SAR antenna to the ground at the first frequency band.
 23. The synthetic aperture radar (SAR) imaging system of claim 21, wherein the multi-band SAR selectively operable in an interrogation mode to transmit a plurality of interrogation pulses to a ground at at least one of a first or a second frequency band, and further comprising: a ground terminal, the ground terminal comprising a ground terminal antenna communicatively coupled to a transceiver, the transceiver communicatively coupled to at least one of a ground terminal data store and a ground terminal data processor, wherein the ground terminal is selectively operable to receive an interrogation pulse and respond by transmitting position information specifying a ground location to the SAR, and wherein the SAR imaging system, in response to receiving position information from the ground terminal, generates one or more SAR images of the ground in response to the interrogation pulse at a first or a second frequency band.
 24. The SAR imaging system of claim 21 wherein the SAR antenna is a planar phased array antenna. 